When the various compounding ingredients are mixed with a polymer to form a rubber compound, it is necessary that the mixed compound be capable of withstanding a limited amount of exposure to heat before vulcanization initiates. This limited exposure to heat is necessary in order to further process the compound by extruding, calendering, and the like, and also to obtain proper mold flow. The compound must flow in order to conform to the shape of the mold before vulcanizing. Rubber compounds are typically formulated to withstand about 5 to 15 minutes exposure at 280.degree. F. before the onset of vulcanization. This time span is commonly referred to as the scorch time of the compound. The onset of vulcanization, which is commonly referred to as scorch, is noted by an increase in the viscosity of the compound. If a scorched compound is further processed, it becomes lumpy and for most applications is unusable.
In a typical styrene-butadiene or natural rubber formulation, the compound will not scorch until after the vulcanization agents are added to the formulation. Therefore, the normal mixing procedure is to add the vulcanization agents near the end of the mixing cycle. If additional time to scorch is desired, post vulcanization inhibitors which function by delaying the action of the vulcanization agents may be used.
The ethylene-higher .alpha.-olefin-polyene (EPDM) polymers have enjoyed wide use in recent years. These polymers have excellent weather resistant properties which leads to their acceptability in applications requiring ozone and heat resistance. The EPDM polymers also have the capability to accept large amounts of fillers such as carbon black, clay, oil and the like. Since increased loading with fillers tends to reduce the cost of a rubber compound, the EPDM polymers are frequently mixed with large amounts of fillers.
In a rubber formulation, carbon black is normally mixed with the polymer at the beginning of the mixing cycle in order to obtain good dispersion of the large amounts of carbon black normally used in the polymer. When EPDM polymers are mixed with substantial amounts of carbon black, premature vulcanization or scorch occurs during further processing and compounding. This scorch occurs even in the absence of vulcanization agents. Since typical commercial vulcanization inhibitors function by delaying the action of the vulcanization agents, they are ineffective in preventing scorch in an EPDM-carbon black mixture where no vulcanizing agents are present. It is desirable to have an EPDM-carbon black mixture which will not scorch.